Hi,
It's recommended (often) you should mix at around 80-85 db. But how do you actually measure how many db's is coming out from your monitors?
Is it possible to look at the specs of my Scarlett 6i6 and my Yamaha HS7 - and determine what my settings should be?
I found various SPL meters but I can't afford one at this moments.
Ideas?

The only way to measure is using a decibel meter set to take a weighted reading. Even a cheap one will get you in the right area.

There are dB measuring apps for tablets and smartphones, how well they work I don't know, but they may be a cheaper option than a hardware meter.

The best audio to measure in my opinion is white noise because its full-frequency. It's also worth moving around a bit and take readings at different spots. Sometimes the room can cause standing waves and frequencies almost silent at one spot can be much louder even a foot away.

Without a meter then try turning up the volume to the point that commercially recorded tracks sound properly frequency balanced. The main thing is to get the volume high enough that our hearing's tendency to focus on the mid range and pay less attention to bass and treble as volume drops (the Fletcher-Munson effect) isn't going to cause a mix imbalance as you boost bass and treble to compensate.

I've found that smart phone apps are usually within +/-3dB of the "right" answer.

Thanks for both helpful answers :-) didn't think of apps, will start by looking into those.

Pair that solution up with either of these free tools, in the box, and you should be good to go! 
 
Just a thought that you may want to balance the relative level of your audio file with the amplification of the speakers, to achieve your desired goal.
 
But maybe you already knew that!  I just looked at your topic and considered that volume and loudness may not be equal in all cases.
 
https://www.kvraudio.com/product/youlean-loudness-meter-by-youlean
 
http://www.meldaproduction.com/MLoudnessAnalyzer

i downloaded a free Db meter onto my smartphone,
and use that to get in the neighborhood

For iPhones, apps can be reasonably accurate since the HW is known - there are only so many models so the SW can do a reasonable job of adjusting to whatever model you have.
 
For other phones, unfortunately it's a question mark so you need to check that the app will give accurate reading given your specific phone.
 
I have one called NoiSee for my iPhone that works well and is within a few dB when I compared it to my HW meter. 

DB reference comparison chart
 
http://www.hearnet.com/at_risk/risk_trivia.shtml
 
Environmental Noise
weakest sound heard     0 dB
normal conversation at 3-5 ft     60-70 dB
dial tone of telephone     80 dB
city traffic inside car     85 dB
 
Musical Noise
normal piano practice     60-70 dB
fortissimo singer 3 ft. away     70 dB
chamber music in small auditorium     75-85 dB
symphonic music peak     120-137 dB
amplified rock music at 4-6 ft.     120 dB
rock music peak     150 dB

White noise is no good.  It does not contain the low end that music does.  Pink noise is the correct noise source for testing SPL when music is involved.  The meter must also be in C Weighting otherwise it ignores the low end which is not what you want.
 
I have a Tandy (Radio Shack) meter sitting up permanently in a very good spot very close to where I am.  It is probably not super accurate either but like many of the options here it gets you right in the ball park.
 
It keeps you honest.  If you dont have one or don't look at it for a while after a few hours you are monitoring at higher SPL levels like creeping up to 90 dB and over.  It is very good for keeping an eye on things.  Some rooms may be loud at 85 dB SPL but I actually think it is perfect level but that is just me.
 
I also still prefer a normal VU type display not digital readouts and I set mine for slow response too so it is has a nice slowish liquid type of movement.  You don't want it jumping around all over the place either.

Saw this while searching for something else, made an account just to respond.
 
Yes, smartphone apps have come a long way!
 
These guys (govt NIH lab) made an app, just downloaded it few days ago, amazing! 
 
Works as good as Pro SLM check out tweet on @NIOSHNOISE (won't allow me to add link)